The Permian volcanic rocks from jianshui area tectonically intrude into southwest part of Shizong-Mile fault belt. It is mainly comprised of basalts、basaltic andesites、andesites and a little picrites. Geological and geochemical characteristics show that they are formed in back-arc environment, matching with Carboniferous pillow lava from Jianshui area. These imply that Yangtze block and Cathaysia block occur plate-subduction in late Paleozoic forming the island-arc volcanic rocks, and that they produce back-arc extension in Permian, then form back-arc basin. From late Paleozoic to early Mesozoic, there is a complex trench-arc-basin system between Yangtze and Cathaysia blocks.